This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. In a randomized, double-blind, placebo-controlled phase II/III trial, to determine the efficacy of 16 weeks of sildenafil therapy on exercise capacity &#40;six-minute walk distance&#41;.